Ultraluminous X-ray source

Ultra Luminous X-ray sources are observed as point-like X-ray sources in nearby galaxies, which are not positioned in the centers of galaxies and have a luminosity which, the Eddington limit for spherical accretion onto a compact object of 10 solar masses with 1038 erg / s in the range of 0 exceeds 3 to 10 keV. You do not win the majority of their energy from thermonuclear reactions. There is no known ultra -luminous X-ray source in the Milky Way. It is believed that the X-ray radiation produced by the accretion of matter onto black holes in X-ray binaries.

Properties

When identifying ultra- luminous X-ray sources other objects are initially excluded as active galactic nuclei behind the galaxy, young supernova remnants in the galaxy and foreground objects such as cataclysmic variables and chromosphärisch active stars in the Milky Way. The verbleibenen X-ray sources have the following properties:

• The X-ray luminosity can be up to 2 * 1042 erg / s reach, and these sources are sometimes referred to as hyper -luminous X-ray sources and only at luminosities 1039-1041 erg / s is spoken of ultra -luminous X-ray sources
• The X-ray radiation is variable with fluctuations in the Internsität and in the spectrum. As with other candidates for accreting black holes in X-ray novae such as transitions between a hard spectrum can be observed with low intensity and a soft spectrum with high intensity. The variability takes place on time scales from seconds to years
• The spectrum in the field of X-ray radiation can be described in first approximation by a simple power law. Below 2 keV, a small excess over the power law is often observed. This excess is interpreted either as a result of an outflow in the form of jets or as emission from the outer cold region of an accretion disk. Above 4 keV breaks the validity of the power laws usually together
• Some ULX with maximum X-ray luminosities of a few 1039 erg show periodic luminosity variations in the order of several hours, such as X-ray binaries of the type LXMB. For ULX with higher luminosities, however, no long-term stable changes in brightness have been observed. As with all types of X-ray binaries dips are also detected in ultra- luminous X-ray sources for which the X-ray brightness decreases by up to a factor of ten within a few hours or days and returned to the previous brightness over the same period. This behavior is interpreted in X-ray binaries as shadowing of the X-rays by structures in the disk or in the stellar wind of the companion star mass -giving.
• Quasi- periodic oscillations of X-rays show frequencies in the range of one to 150 mhz, as they are also observed in X-ray binaries with black holes. The distribution of the frequencies of the QPOs have a high mass black holes expected
• The ultraviolet radiation from the place of the ultra -luminous X-ray sources sometimes show signs of accretion of the stellar wind of a Wolf- Rayet star and sometimes signs of a mass transfer when a star in a binary system exceeds its Roche limit
• Optical radiation from the place ultra- luminous X-ray sources has usually over a blue color index. However, this must not be interpreted as a massive blue star as the source of the accreted gas, since it can also involve radiation of the accretion disk, which is heated by the X-rays. Since the reprozessierte radiation of the accretion disk exceeds the optical brightness of the star by up to 5 magnitudine can only companion with spectral type O are excluded
• To some ultra -luminous X-ray sources ionized nebulae are found, their extent and luminosity that significantly exceeds that of supernova remnants. The lines of He II and [ Ne V] are also observed in active galactic nuclei, where they are interpreted as a result of photoionization by hard X-rays
• Ultra Luminous X-ray sources are found to be 75 percent in or near areas with active star formation in the galaxies. Accordingly, these X-ray sources are rarely discovered in elliptical galaxies. In recent years, some young open clusters or globular clusters have been found with an age of around 10 million years at the place of the ultra- luminous X-ray sources. Since the rates of star formation in interacting galaxies are quite high, a correspondingly high number of ultra- luminous X-ray sources are observed there. The stars in the vicinity of these ULX usually have an age on the order of 10 million years
• The Ultra Luminous X-ray sources are more likely to occur in metal-poor galaxies with a proportion of heavy elements, which is less than five percent of the value of the sun
• A subset of the ultra- luminous X-ray sources are the super soft ultra- luminous X-ray sources, the X-ray spectrum of radiation with energies below 2000 electron volts is dominated. These spectra can be described both as accretion disks around black holes with high inclination with stellar masses as well as cool disks around black holes with masses of 1,000 to 10,000 solar masses

Classification

Ultra Luminous X-ray sources are divided into based on the luminosity in X-rays

• In the 1039-2 * 1040 erg / s as standard Ultra Luminous X-ray sources or sULX
• Until 1041 erg / s as extreme ultra -luminous X-ray sources or eULX
• For values ​​beyond 1042 erg / s as hyper -luminous ULX or HLX

In addition to the luminosity of the classes also differ in their X-ray spectrum and in its quasi-periodic variability.

Hypotheses

The interest in the ultra- luminous X-ray sources located at the crossing of the Eddington limit for known companions in X-ray binaries. Each star can then gain a maximum luminosity from the accretion before the radiation pressure wegbeschleunigt the incident matter of the compact object, thus placing an upper limit of the accretion rate and luminosity for each mass. Therefore, the following hypotheses have been developed for the ultra- luminous X-ray sources:

• It is accreting black holes with masses of more than 100 solar masses. This hypothesis is often seen skeptical, because there is no other evidence for black holes with a mass above 20 stellar masses, which may arise directly from a core-collapse supernova. Only black holes with masses well above a hundred thousand or more solar masses have been found in the active nuclei of galaxies. Black Holes intermediate-mass may have arisen from the merger of black holes with stellar mass
• It is accreting black holes with masses of five to 20 solar masses, whose X-ray radiation is not isotropic radiated, but the emission is highly directional.
• It is accreting black holes with masses of five to 20 solar masses, the radiation exceeds the Eddington limit. A slight excess of the Eddington luminosity is known of the galactic microquasars V4641 Sgr and GRS 1915 105.
• It is accreting black holes with masses of up to 80 solar masses, which have arisen from extremely hydrogen- poor stars and just emit above their Eddington luminosity. At low metallicity only small stellar winds and therefore lose mass stars in their last stages of star less matter before they explode as core-collapse supernova and a black hole remains.
• A hybrid model of such alternatives

In addition to these hypotheses to unusual X-ray binaries are also discussed, whether it is a

• Could act a former central black hole of a now verschmolzenden with the smaller galaxy galaxy
• It could be either of type IIn supernova. However, never an optical counterpart of a supernova has been observed at the site of an ultra- luminous X-ray sources
• It could be a black hole at the center of the galaxy ricocheting smaller black hole that has been accelerated to velocities of a few 1000 km / s
• Could involve young rotationally driven Röntgenpulsare some 100 to 1000 years after the supernova explosion. For the X-ray pulsar wind nebula brightness also can contribute. It is estimated that approximately three percent of the ULX are driven by the rotational energy of young neutron stars

Probably ultra- luminous X-ray sources are not a homogeneous group. In particular, the hyper -luminous X-ray sources show different properties as a harder X-ray spectrum and greater variability in the range of seconds to hours. They are considered good candidates for accreting black holes with average masses in the range from several hundred to 10,000 solar masses.

Examples

• XMMUJ004243.6 412519 in M31
• HLX -1 in ESO 243-49
• CXOU133705.1 - 295 207 in M83

Itemization

• Ray binary